This invention relates to the improvement of an electron beam patterning method and apparatus, and more particularly to the correction of deflection distortion.
In the electron beam patterning apparatus since the deflection distortion of the electron beam greatly affects the precision of pattern formation, some correction therefor must be made. For example, as disclosed in U.S. Pat. No. 3,900,736, actual deflecting signals for deflecting the electron beam are used which satisfy the following equations EQU U.sub.X =AX+BY+CXY+D (1) EQU U.sub.Y =EX+FY+GXY+H (2)
where U.sub.X and U.sub.Y represent the actual deflection signals in the X-axis and Y-axis directions, respectively, and A to H are the correction factors. The correction factors A to H are determined as follows.
On an object or sample which is irradiated by an electron beam, there are formed a number of reference marks for permitting the electron beam to be directed to a correct position. One of the reference marks is moved to the vicinity of an area in which the deflection signal is zero or to a region where the deflection distortion is negligible (referred to as region of near zero deflection signal), the electron beam is caused to impinge upon the object, and the reflected electrons or secondary electrons are then detected. The detected output is read into a control apparatus where computation is performed to determine the position of the one reference mark.
The closest reference mark adjacent to the above one reference mark is similarly moved, by moving the object, to the region of near zero deflection signal, and detection is made thereat. The detected position and the moving distance of the object are determined and read into a control apparatus to determine the distance between the two adjacent reference marks. In this way, the distances between all the reference marks are determined as a first step.
Then, the object is moved so that the central one of the reference marks can be detected at the region of near zero deflection signal. After the movement, the remaining reference marks surrounding the central reference mark are detected by deflecting the electron beam and the positions thereof are computed by the control apparatus. This is a second step.
The difference between the positions of the reference marks which are computed at the first and second steps is the deflection the distortion caused by deflecting electron beam. In order to remove the deflection distortion, the correction factors A to H in the equations (1) and (2) are changed and then established in a deflection distortion correcting circuit. Then, the above second step is repeated several times until suitable correction factors can be determined to provide zero difference between the positions of the reference marks.
However, this method inevitably produces more or less, asymmetrical and independent distortion in each quadrant because the precision of fabrication of a deflector system such as deflector coil has a limitation. Such a distortion can not be expressed by the equations (1) and (2) and cannot be properly corrected.